The efficiency of a capacitor is measured by its dielectric constant. Although capacitor design has been in existence for several hundred years with the invention of the Leyden jar, miniaturization of modern electronics has put increasingly greater demands on the improvement of capacitor design because capacitors protect integrated circuits from spurious electrical signals and damp out surges that would otherwise damage the circuits or disrupt their operation.
Because of the nature of ceramic material, the behavior of the electrons in that material offers increased capacitance and efficiency when used as an insulating material. Even though the electrons in the insulator are not mobile, they can shift slightly under the influence of the attractive and repulsive forces from the electrodes on each side. Electrons on one side of the insulator bulge out from the surface creating a negative charge while on the other side they retreat from the surface giving rise to a positive charge. The charges thus generated on the insulator help to neutralize the charges on the electrodes. Some insulators can bear charges that are nearly as large as those on the electrodes themselves. Such neutralization reduces repulsive forces and allows more charge to reside on the electrodes which in turn increases capacitance which is reflected in the dielectric constant. It is this property which accounts for the efficacy of the multi-layer ceramic capacitor.
In multi-layer ceramic capacitors (MLC's) ceramics generally consisting of barium titanate with small amounts of other oxides are generally used as the insulator. The ceramics have dielectric constants of between 2000 and 6000 and are in the form of fine powders that have particles a few micrometers in diameter. The particles are dispersed in a solvent resulting in a slurry with the consistency of paint. The slurry is then cast in thin sheets onto a paper or stainless steel belt with the sheet thickness controlled by a blade. The slurry dries as the solvent evaporates leaving a smooth unfired or green tape which is cut into squares from 6 to 8 inches square. Thousands of electrodes are printed on each sheet through a thin screen that delineates the electrode pattern. The general structure and assembly of such a capacitor is described in the July 1988 issue of Scientific American at page 86 et seg.
MLC's are very sensitive to temperature, as a result of which there is the problem of capacitance drift. Another limitation of the MLC is its operating temperature limit. That limit is 125.degree. C. to 150.degree. C. which can be a severe limitation in certain aerospace applications. Consequently, to be able to obtain the advantages of using the MLC, removal of the heat created by capacitor losses so as to keep the temperature and thermal shock within operating range is a critical factor for circuit design. However, it is also undesirable to overcool the capacitor because this will increase the heat losses due to the inverse relationship between temperature and heat loss.
In the past, it has been proposed to dissipate heat within a capacitor in several ways. For instance, U.S. Pat. No. 1,474,486 discloses a capacitor having particular application for transmitters and radio communications in which metal foil conductors and mica insulators are successively joined in a stack. Each pair of adjacent sections comprising a plurality of alternately disposed mica and foil sheets are separated from each other by two insulating sheets which on one side may be of mica and on opposite sides of a conductor sheet which conducts both electricity and heat. Although this arrangement is designed to promote heat radiation, it is not directed to the particular construction of MCL's and, more importantly, to the properties of MCL's which require a higher efficiency cooling arrangement.
U.S. Pat. No. 1,713,867 shows a similar arrangement in which end blocks of metal are placed against a section of mica sheets which serves to strengthen the sheet and radiate heat from the section. However, the metal end blocks are contained within the periphery of the mica sheets and would not be satisfactory for conducting capacitance losses generated by MCL's.
U.S. Pat. No. 3,840,780, relates to a wound capacitor comprising alternate strips of paper and electrode foil. A resistor strip is placed near the end of the capacitor roll section for controlled dissipation of any undesirable voltage existing between the capacitor terminals and also, to some extent, to dissipate heat within the capacitor. Again, however, such an arrangement would not be satisfactory to keep an MCL within its operating limit of 125.degree. C. in circuits used for certain applications such as high speed switch mode power supplies which require extremely low equivalent series resistance and equivalent-series inductance capacitors for output filtering.